1. Field of the Invention
The present invention relates to a liquid crystal display device.
2. Description of the Related Art
Conventional active matrix liquid crystal display devices include a type called semi-transmissive reflective type. A semi-transmissive reflective liquid crystal display device can realize both the system of turning on a backlight disposed at the side of the back surface of a liquid crystal display panel to allow an optical image that passes through the liquid crystal display panel to be viewed, and the system of turning off the backlight but irradiating external light from the side of the front surface of the liquid crystal display panel to allow an optical image that is reflected on a reflective layer formed in each pixel electrode formed on the liquid crystal display panel to be viewed. A liquid crystal display device described in Unexamined Japanese Patent Application KOKAI Publication No2003-222854 is known as a liquid crystal display device of this type. In the liquid crystal display device described in this publication, scanning lines (including the gate electrode of thin film transistors as switching elements) and reflective films which are both made of aluminum-based metal are formed on the upper surface of an active substrate, a gate insulating film is formed above these scanning lines and reflective films, the portions other than the gate electrode of the thin film transistors and data lines are formed on the upper surface of the gate insulating film, pixel electrodes whose area is larger than that of the reflective films are formed on the upper surface of the gate insulating film above the reflective films, the regions where the reflective films, which positionally overlap with the pixel electrodes, are arranged serve as reflective pixel regions, and a most part of the region of each pixel electrode other than the region where the pixel electrode and the reflective film positionally overlap, serves as transmissive pixel region.
However, in a case where this conventional liquid crystal display device is used as reflective type, external light that enters from the side of the display surface passes through the pixel electrodes and the gate insulating film to be reflected on the reflective films, and this reflected light passes through the gate insulating film and the pixel electrodes to be let out to the side of the display surface. Accordingly, since the external light passes through the pixel electrodes and the gate insulating film twice, there is a problem that the reflectivity drops due to such phenomena as absorption, interface reflection, interference phenomena, etc., and the displayed image is unfavorably colored.
Further, in a case where a liquid crystal display device as described above includes auxiliary capacitor electrodes, which are formed on the upper surface of the active substrate, and whose regions that positionally overlap with the pixel electrodes are to be used as reflective films, the area of the region of each auxiliary capacitor electrode that overlaps with the pixel electrode constitutes an auxiliary capacitor forming area and reflective area. However, since these areas often take different values, there is a problem that the degree of freedom in designing is limited.
Furthermore, in a case where the above-described liquid crystal display device has, on the upper surface of the active substrate, lower lines which are made of aluminum-based metal, the same material as the scanning lines and reflective films, and has, on the upper surface of the gate insulating film, upper lines which are made of chromium-based metal or the like, the same material as the data lines, and which are connected to the lower lines through contact holes formed in the gate insulating film, the surface of the lower lines made of aluminum-based metal is very easily oxidized. Since this promotes formation of a native oxide film having a high resistance on the surface of the lower lines, there is a problem that the contact resistance becomes unstable.